They just have a sped up clip going really fast as they put up the title. You can see the movement of the people in the background all moving fast. However long it took for the guy or gal to race, they took that whole clip and sped it up to get that effect, or blur.

In Shotcut, on the timeline, split where you want to start, split where you want to stop. Click on the clip in the timeline, then go to Properties, then speed up the clip.

You don’t have to use the timeline, as you could make a clip and put into the playlist, then speed it up. Just my perspective as I prefer using the timeline.

Sorry I wasn’t clear enough. I meant specifically how to frame blend / motion blur a sped up video.

For example, the first 8s has frame blend on, the other half is the same clip with it off. (gopro studio)
Both clips are the same speed (70x) but one looks smooth due to the blur and the second one looks crazy without the blur… the 2nd one make it look like 30 cars! The first one makes it look like fast cars in a timelapse

I don’t just want to speedup, I’d like to motion blur / frame blend the speed up

This is not possible in Shotcut. I will move it to Suggestion category, but do not expect it anytime soon. Meanwhile, there is another open source program called slowmoVideo that can do this. Maybe someday we will merge that program into Shotcut.